Power reserve, winding, magnetism and servicing are common possibilities. The exact cause depends on the watch, its movement, age, case construction and recent history.
For why does my automatic watch keep stopping, a visual inspection and basic diagnostic checks usually give a better answer than guessing from the symptom alone. Avoid forcing crowns, pushers or case backs if something does not feel right.
Where repair is appropriate, the work and price should be explained before chargeable work begins. Parts availability, water resistance and originality can all affect the recommended approach.
Need a professional assessment?
If the watch is showing similar symptoms, start a repair and describe what you have noticed. Avoid forcing the crown or controls if the watch appears damaged.